Multi-colored Cancels. 
Most ordinary mail cancels are in black ink. Official mail and Paid mail are normally cancelled in red. When official mail is mail from a small office, they are often found in black, although the rules call for red ink. This is because small offices cannot dedicate a cancelling machine to red ink, and changing ink color is messy and time-consuming.
In 1976, the government sought to decentralize functions, and move jobs out of the Dublin area to areas where there was chronic unemployment. The Department of Lands opened an office of Fisheries and Forestry in Castlebar, County Mayo. This made the Castlebar Post Office the first office outside of Dublin handling significant amounts of official mail.
Cancelling machines require meticulous cleaning when changing ink colors. Not only must the ink reservoir be changed, but the rollers and ink pads must cleaned or replaced. Failure to do so will result in carry-over of colors resulting in multicolored cancels.
The obvious solution is to dedicate machines to a particular color, however, it was sometimes necessary to change colors, thus adding a little color into someone’s collection.